My records show a set of two one barrel carbs on the GMC 616 CID 6 cylinder inline used on the model Z Yellow Coach (bus) from 1931 to 1934; but they were downdraft carbs (Strombergs).

Cadillac used dual updraft one-barrels on the V-12 and V-16 engines from 1931 through 1937, but of course these were not inline engines.

I have no record of any dual single-barrel updraft manifold used by GM on an inline engine.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop